456 
Theory  of  Indicators,  etc. 
(  Am.  Jour.  Pharm. 
1     October,  L904. 
Attention  has  already  been  drawn  to  the  fact  that  indicators  are  for 
the  most  part  acids  ranging  from  those  in  the  first  group  possessed 
of  considerable  affinity  for  alkalies,  and  consequently  more  suitable 
for  the  titration  of  bases,  to  those  of  the  third  group  which  have 
the  weakest  acid  properties,  and  are  consequently  capable,  as  has 
been  explained  above,  of  being  employed  as  indicators  for  the  titra- 
tion of  dilute  acids,  provided  strong  bases  are  made  use  of  in  the 
standard  solutions.  We  have  already  considered  the  modus  oper- 
andi of  the  third  group.  As  we  ascend  the  scale  to  the  second 
group  possessed  of  more  marked  acid  characteristics  it  necessarily 
follows  that  the  indicators  of  this  group  are  less  sensitive  to  weak 
acids  than  are  those  of  the  third  group,  owing  to  the  fact  that  the  acid 
properties  of  such  indicators  are  more  nearly  equivalent  to  those  of 
the  weak  acids  which  have  to  be  titrated.  They  are,  however,  still 
capable  of  reacting  with  strong  mineral  acids,  and  are  more  sensi- 
tive to  bases  than  are  the  weak  acid  indicators  of  the  third  group. 
Phenolphthalein  and  Porrier's  blue,  for  example,  members  of  the 
third  group  which  may  be  employed  for  titrating  the  weakest 
organic  acids,  give  sharp  end  points  with  soda  and  potash,  but  not 
with  ammonia.  Hematoxylin,  belonging  to  the  second  or  middle 
group,  is  capable  of  giving  a  sharp  end  point  on  titrating  with 
ammonia,  but  cannot  be  employed  for  estimating  the  amount  of 
weak  organic  acids  in  a  solution. 
As  we  pass  from  the  second  to  the  first  group  of  indicators  the 
acid  properties  become  even  more  marked.  These  indicators  are 
for  the  most  part  sulphonates  of  diazo  compounds  and  possess  sev- 
eral OH  groups  in  addition.  They  are  sufficiently  strong  acids  to 
form  salts  with  relatively  weak  bases ;  but,  on  the  other  hand,  owing 
to  their  strongly  acid  characteristics,  are  quite  indifferent  to  the 
presence  of  weak  organic  acids  in  solution.  We  have  in  the  solu- 
tion a  weak  base  to  be  titrated,  also  a  small  quantity  of  the  salt  of 
the  indicator  acid.  The  color  change  is  brought  about  when  the 
weak  base  being  titrated  has  been  completely  neutralized  by  the 
acid  employed  for  titration  and  the  slight  excess  of  the  acid  added 
results  in  the  liberation  of  the  indicator  acid  from  its  salt.  Differen- 
tiation between  bases  of  different  strength  by  means  of  indicators 
of  the  first  group,  whilst  not  so  readily  effected  as  is  that  of  acids  in 
the  third  group,  is  still  perfectly  feasible.  Some  of  the  indicators  of 
the  first  group,  as  for  example,  phloroglucinvanillin,  depend  for 
